A circuit-carrying substrate used in a computer or in peripheral equipment thereof includes electronic components such as transistors and an electric component such as a motor. The substrate is housed in an airtight case and electrical terminals connected to the substrate circuitry extend out of the case in a connector for connection to components outside the case.
FIG. 8(a) of tie accompanying drawings is a perspective view of a prior art airtight case and FIG. 8(b) is a longitudinal sectional view taken on line b--b of FIG., 8(a).
This known airtight case 100 comprises a housing 70 having an opening and a cover 80 made from a metal, plastics material or the like for closing the housing opening. A plurality of electrical terminals 74 extend out of the case 100 in a connector 72 which projects outwardly from one side wall of the housing 70. The terminals 74 of the connector 72 provide electrical connection between the circuitry of a substrate 50 fixed inside the housing 70 and components outside the case 70.
After the substrate 50 has been mounted inside the housing 70, a sealing packing 90 is interposed between the rim of the opening of the housing 70 and the cover 80, and the housing 70 and the cover 80 are secured together by fasteners, such as screws 60.
The resulting airtight case 100 must be completely waterproof to prevent water from entering the inside of the case.
Thus, if the inside of the airtight case 100 communicates with the outside air, the conditions of the outside air, such as the temperature and moisture of the outside air, may affect the substrate 50 inside the case 100. In particular, if humid outside air enters the inside of the airtight case 100, metal portions such as the circuitry of the substrate 50 may be corroded by the water contained in the outside air and thereby cause an electrical contact failure.
The airtight case 100 must therefore have excellent air-tightness.
When an electric or electronic part inside the airtight case 100 generates heat and the temperature inside the airtight case 100 rises, the pressure in the case 100 increases due to the expansion of the air within the case. Similarly, depending on the conditions, the pressure inside the airtight case 100 may become lower than atmospheric pressure.
To equalize such pressure differences between the inside and outside of the airtight case 100, through holes may be formed in the airtight case 100.
However, if through holes are formed in the case 100, the above waterproofness and air-tightness are inevitably impaired.
To cope with this, it has been proposed to use an airtight case of the form shown in FIG. 9 of the drawings. In the airtight case 102 shown in FIG. 9, a through hole 704 opening into the connector 72 is formed in the side wall of housing 702 of the case 102 and a sealing packing 902 is provided between the connector 72 and the connector 72' of a component D to which the connector 72 is connected to prevent the through hole 704 from being exposed to the outside air. In another known arrangement shown in FIG. 10, a through hole 714 formed in the side surface of a housing 712 of an airtight case 104 is covered with cloth 716 serving to transmit only air and not moisture. These prior art structures attempt in this way to eliminate any pressure difference between the inside and outside of the airtight case 102 or 104 whilst maintaining the waterproofness and air-tightness of the case.
However, as will be clear from FIG. 9, even with the case 102 having the through hole 704 opening into the connector 72 and the packing 902 around the connector, it is impossible to retain air-tightness completely and thus impossible to prevent outside air containing moisture from entering the inside of the airtight case 102.
Further, as shown in FIG. 10, if the rough hole 714 of the airtight case 104, covered with the cloth 716 for transmitting only air and not water, becomes blocked with water, it no longer functions as a pressure control hole.
As described above, it is therefore difficult using the airtight cases 100, 102 and 104 of the prior art to eliminate a pressure difference between the inside and outside of the case while retaining the air-tightness of the case.
When it is also necessary to use the sealing packing 90 or 902, there is a further problem in that the number of parts and the production cost is increased.